Research and Development Methods

Matrix Isolation Electron Spin Resonance (MIESR)

A. Basic Principle:

The theory of electron spin and the related concept of magnetic dipole moment of the electron were postulated in 1925 in order to explain observed effects in the magnetic and x-ray spectra of certain atoms (Rich, 1992). The spectra appeared in the form of distinctive wavelength features of emitted resonance electromagnetic radiation when the molecules or atoms were subjected to magnetic flux conditions using magnetic flux instruments. Although theories use concepts such as "orbit" and "axis" to graphically describe the atomic structure, no similar theory exists that enables a simple pictorial depiction of electron spin. Nevertheless, application of spin resonance instruments and techniques enables the identification of atoms (and molecules, including radicals) and comprehension of their precise structure (Weissman, 1992).

Electron spin resonance analysis begins with the insertion of the atoms or molecules in a liquid solvent or cryogenic material (the matrix), and the use of a wide range of materials and temperatures has been reported. For example, o-Terphenyl was used at 275ºC (Lewis and Singer, 1981), deuterium oxide (D₂O) at 77 K (Mihelcic et al., 1993), and Tetrahydrofuran at 203 K (Gescheidt, 1994). Then, the matrix is fed into a spectroscopic detector that detects the magnetic resonance emission. The term "matrix isolation electron spin resonance" refers to the two stages of the ESR technique. Thus MIESR is not generally considered a remote sensing method. Nevertheless, its capability for precise atomic analysis has led to the transportation of equipment to remote areas and use in programs as a comparison method with other technologies (Zenker et al., 1998). Comparison of this method with other instruments are given by Crosley (1995) and Werst and Trifunac (1998).

The range is the typical span of species concentration that would be measured by the MIESR technique. The concentration ranges shown in following table are the atmospheric concentrations that had been measured previously; they are not the span of a specific MIESR instrument.

C: Minimum Detection Level:

The minimum detection level is the lowest concentration of a species that instruments using MIESR technique can detect.  MIESR detection limits are typically in parts-per-trillion by volume level.
 

D. Operating Temperature:

The application of MIESR for analysis of ambient air has been developed as a two-stage process: First, ambient air is sampled by reduction to a very cold temperature; then, the frozen sample is transferred to the input state of the spectrometer. Ambient air is drawn into a vacuum chamber where it comes in contact with a very cold surface at about 77K. As the free radicals in the air sample adhere to the cold surface, a separate supply of some liquid such as water or D₂O is allowed to build up on the cold surface.

The cold surface is then stored cryogenically, while transported to the MIESR spectrometer. Since the radicals are extracted from the incoming ambient air onto the cold surface, the incoming air can be at any temperature. The air sampling instrument can operate in ambient air conditions, but the spectroscope need not do so. Ambient air sampling has been conducted at Schauinsland during the nighttime in August, where the estimated ambient air would be 8°C, average minimum (Rudloff, 1981), and at Tenerife (Canary Island) in August where the minimum average ambient air temperature would be around 19°C (Rudloff, 1981).

Interferences are factors or conditions, either chemical, meteorological, or instrumental, which could be responsible for introducing error into the measurement results from use of MIESR.

1. MIESR method involves "freezing" (i.e., isolation) of many of the species mentioned above in a non-reactive matrix enabling their relative concentrations to be determined (Werst and Trifunac, 1998). One problem in the procedure is the matching of probable reactions to measured concentrations, sometimes called the "fake NO", which requires the assumption of more or less of some species than found by actual measurement (Mihelcic et al., 1993)
2. Even with the matrix isolation technique and the development of numerical techniques for MIESR spectral analysis, the separation for some species, e.g., the alkylperoxy radicals, has not been successful because of the close similarities of their spectra (Mihelcic et al., 1990).
3. Variability of MIESR was found to be about 50 percent in the measurement of peroxy radicals (RO_x) (Zenker et al., 1998).

The following notes of interest are possibly useful facts in the development of any strategy involving MIESR:

1. The notion of matrix isolation has typically implied very cold surfaces on which the radicals are trapped in a rare gas or halocarbon matrix, such as the Freon. However, halocarbon matrices which are liquids at room temperatures have also been used (Werst and Trifunac, 1998).
2. One of the conclusions of the intercomparison report by Zenker, et al. (1998) is that the comparison of methods measuring NO₂ gives some confidence that accurate measurements in the 50 - 700 pptv range can be achieved. The three methods compared were MIESR, chemilumescence, and tunable diode laser absorption spectroscopy. The differences were in the range of 4-13%.
3. Although the numerical analysis of the MIESR spectra has not resolved all of the problems, the principle has also been applied in the analysis of heavy metal pollutants in the atmosphere (Iosefzon-Kuyavskaya et al., 1993).

1. Crosley, D. R., The measurement of OH and HO₂ in the atmosphere: J. Atmos. Sci., 52(19):3299-3314, October 1995.
2. Fischer, H., et al., Trace gas measurements during the oxidizing capacity of the tropospheric atmosphere campaign at Izana: J. Geophys. Res., 103(D11):13505-13518, 1998.
3. Gescheidt, G. A., A simple experimental setup for the simultaneous measurement of ESR and absorption spectra: Rev. Sci. Instru., 65(6):2145-2146, June 1994.
4. Iosefzon-Kuyavskaya, B., et al., ESR indication of heavy metal contamination of urban atmosphere: Water Sci. Technol. 27(7-8): 263-269, 1993.
5. Lewis, I. C., and L. S. Singer, Electron spin resonance study of the reaction of aromatic hydrocarbons with oxygen: J. Phys. Chem., 85(4): 354-360, 1981.
6. Mihelcic, D., et al., Numerical analysis of ESR spectra from atmospheric samples: J. Atmos. Chem. 11:271-297, 1990.
7. Mihelcic, D., et al., Simultaneous measurements of peroxy and nitrate radicals at Schauinsland: J. Atmos. Chem., 16:313-335, 1993.
8. Penkett, S. A., J. Geophys. Res., 103(D11): 13353-13355, June 1998.
9. Rich, A., Electron Spin: McGraw Hill Encyclopedia of Science and Technology. The 7^th ed., New York: McGraw Hill Inc., 1992, 6:218-220.
10. Rudloff, W., World-Climates, 1981, Stuttgart: Wissenschaftliche Verlagsgesellschaft mbH.
11. Weissman, S. I., Electron paramagnetic resonance (EPR) spectroscopy: McGraw Hill Encyclopedia of Science and Technology. 7^th edition, New York: McGraw Hill Inc., 1992, 6:207-210
12. Werst, D. W. and A. D. Trifunac, Observation of radical cations by swiftness or by stealth: Accounts of Chemical Research, 31(10): 651-657, 1998
13. Zenker, T., et al., Intercomparison of NO, NO₂, NO_y, O₃, and RO_x measurements during the oxidizing capacity of the tropospheric atmosphere (OCTA) campaign 1993 at Izana: J. Geophys. Res., 103(D11): 13615-13634, June 1998